Earth Sciences 2014; 3(3): 85-96

Published online July 10, 2014 (http://www.sciencepublishinggroup.com/j/earth)

doi: 10.11648/j.earth.20140303.13

ISSN: 2328-5974 (Print); ISSN: 2328-5982 (Online)

Cenozoic eruptive stratigraphy and structure in Taiz area of Yemen

Abdul-Hamid Malek1, Mysore Ramachandraiah Janardhana

2, Abdul-Aleam Ahmed Al-Qadhi

2, *

1Department of Geology, Faculty of Applied Science, Taiz University, Taiz, Yemen 2Department of Earth Science & Resources Management, Yuvaraja's College, University of Mysore, Mysore-570005, India

Email address: a_malek74@yahoo.com (Abdul-Hamid M.), drmrjanardhana@rediffmail.com (M. R. Janardhana),

abdul.aleam@yahoo.com (Abdul-Aleam Al-Qadhi)

To cite this article: Abdul-Hamid Malek, Mysore Ramachandraiah Janardhana, Abdul-Aleam Ahmed Al-Qadhi. Cenozoic Eruptive Stratigraphy and

Structure in Taiz area of Yemen. Earth Sciences. Vol. 3, No. 3, 2014, pp. 85-96. doi: 10.11648/j.earth.20140303.13

Abstract: The present study focuses on the field description of a bimodal volcanic rock centre and fault types present in Taiz area located in the southern part of Yemen. Taiz area serves as one of the key areas to understand the Afro-Arabian

bimodal volcanism and the emplacement of Afar plume and the relationship between extension tectonics and magmatism.

Taiz area comprises bimodal volcanic rocks encompassing mafic and silicic lava flows and pyroclastic rocks. The lava flows

were erupted in five phases – three major basic and two acid phases. Lower basalt sequence (Tb1) is the oldest and main

phase of flood basalt volcanism, formed as part of a wide spread volcanism within the Afro-Arabian region. This is followed

by lower silicic sequence phase (Tr1) consisting of varied assortment of lithologies such as rhyolite flows with subordinate

ignimbrites, welded ash, pyroclastic breccia, and random pumice and obsidian. The middle basalt sequence phase (Tb2) has

been formed from the flows fed by fissure-dyke systems and separated from Tr1 by red colored weathering band of saprolitic

bole (paleosol). The latest silicic sequence phase (Tr2) with limited exposures in the region, comprises rhyolitic plug domes,

rhyolitic lava flows and locally welded and unwelded volcaniclastic deposits. In places, the silicic volcanic rocks have been

diagenetically altered to bentonitic clay minerals and zeolites. The youngest phase of volcanic rocks represented by the

basaltic rocks in the region (Tb3) is exposed in few outcrops along the southeastern slope of the mapped area and

conformably capping the Tr2. It includes the basaltic flows intercalated with mafic conglomerate and tuff layers. The study

area has been subjected to tensional tectonic regime throughout much of the Tertiary and extensions led to volcanism, granitic

rock intrusions and formation of structural elements such as normal faults and deep joints. NW, NE, and E-W are the three

major trends of faults recognized and these are related to the progressive rifting of Red Sea and Gulf of Aden.

Keywords: Yemen Volcanic Group (YVG), Taiz Area, Basalt, Bimodal Volcanism, Saber Granite

1. Introduction

Cenozoic Era is known for intense extrusive and

explosive volcanic activities in isolated parts of the globe

and development of many volcanic landforms. Across the

globe, the Cenozoic volcanism is related to the movement of

the lithospheric plates and largely associated with either

subduction or collision tectonics. Prominent examples are

the volcanism; on the Tibetan plateau in response to the

India-Asia continental collision [1, 2] related to the Pacific

plate subduction, back-arc spreading and corresponding

marginal continental rifting in the eastern marginal part of

Eurasian plate [3], in coastal California - erupted to the west

magmatic arc trend related to subduction along the

continental margin [4] and volcanism forms a magmatic arc

running along the western margin of Sardinia and southern

corse microplates in Italy [5]. However in Yemen, the

Cenozoic volcanic activity in the region is related to the

uplift of the Afro-Arabian plate [6] and subsequent break up

into separate blocks as a consequence to the up rise of the

mantle plume head called Afar plume [7, 8]. This process led

to lithospheric extension and emplacement of thick bimodal

volcanism with silicic pyroclastic rocks, intrusive bodies,

and dyke swarms associations in Ethiopia, Djibouti, and

Yemen. The huge pile of volcanic rocks on both sides of

Red Sea reflect a tensional tectonic setting within

continental crust associated with the opening of the Red Sea

and Gulf of Aden during the early Oligocene (31 to 26 Ma)

Earth Sciences 2014; 3(3): 85-96 86

[9-13]. The 31 Ma basaltic and 29.7 Ma silicic eruption

episodes began the construction of Yemen Volcanic Group

(YVG), [14] which generally overlies Late Jurassic

carbonate shelf sediments called Amran Group and clastic

material termed Tawilah Group. Previous studies have

contributed significantly to the knowledge of the surface

geology including mapping [15, 16] of the volcanic rocks in

the region. However data on the volcanic stratigraphy and

structure of the region is scanty. Little is known in detail on

the Cenozoic volcanic succession in the Taiz area. This

paper describes in detail the lithology and stratigraphy of

early Cretaceous volcanic rocks of Yemen Volcanic Group

in Taiz area thereby provides a complete record of eruption

and eruption frequency over a particular time span in the

studied area.

2. Volcanology of Yemen

The western and central part of Yemen is an uplifted

highland and plateau (platform) bordered by Tihama plain in

the west and Marib-Balhaf graben in the east. The

stratigraphic sequence of uplifted regions includes rocks

ranging in age from the Archean to Cenozoic [17,18]. The

Precambrian rocks are exposed in several isolated blocks in

the north and east of uplifted highland and consists of

basement gneisses and island arc associations (Fig. 1)[19].

The Paleozoic (Wajid and Akbarah Formations) and

Mesozoic (Kuhlan formation, Amran and Tawilah Groups)

sediments rest unconformably on the Precambrian rocks

[20]. As streaming lava erupted along the geological fissures,

the Precambrian base and Mesozoic sediments were covered

by layers of various types of Cenozoic volcanic rocks,

especially in the southwestern part of uplifted area as part of

Afro-Arabian flood volcanic succession. The Afro-Arabian

flood volcanic succession consists of an isolated magmatic

sub-regions distributed in sinuous line extending from

Kenya in south to Syria in the north passing through

Ethiopia, Djibouti, Eritrea, Yemen, Saudi Arabia and Jordan

(Fig. 2) [14]. The succession of the volcanic sub-regions is

composed of basaltic lavas, rhyolitic ignimbrites and

pyroclastic fall deposits, in addition to less common basaltic

pyroclastic rocks and rhyolitic lavas [21-24, 13, 14]. In

Yemen, the Cenozoic volcanic group was originally named

the Yemen volcanic group (YVG) [25, 18, 26]. The YVG

consist of thick sequence of volcanic flows and forms the

largest sub-region of Tertiary volcanic rocks in the Arabian

plate. The volcanic rocks cover many thousands of square

kilometers of the Yemen and extend across the Red Sea into

Ethiopia (Fig. 2). The total estimated volume for the YVG is

greater than 40,000 km3 of bimodal basalt/rhyolite

volcanism [27], with the maximum thickness of over 2000 m

occurring in the western part. The rocks were accumulated

on an irregular pre-volcanic terrain underlain by Paleozoic

and Mesozoic sedimentary rocks and Precambrian

crystalline rocks. The YVG can be divided into the late

Oligocene - early Miocene Yemen Trap Series (YTS),

separated by an unconformity from the late Miocene Yemen

Volcanic Series (YVS) [25]. YTS consist of

north-northwest-trending linear fissure systems, extending

from long. 43o30' to 44o55' N and lat. 13o to 15o 30' E [18]

covering an area of about 40 km2.

Figure 1. Simplified geological map of Yemen (modified after Garzanti et al.

2001) showing the relationship between main rock types and principal

grabens. The bold-line rectangle marks the location of the Taiz area.

Figure 2. Simplified regional map showing distribution of Cenozoic

volcanism in East African and Arabian plates related to Red Sea and Gulf

of Aden opening. Note location of plume head upwelling (modified after

Davidson and Rex 1980 and Ukstins Peate 2005).

YTS comprise part of a larger flood volcanic province

extending into Eritrea and Ethiopia [27].The eruption of

YTS took place during a period from about 31.6 to 15 Ma

[25, 28] although volcanic activity may have commenced as

early as 40 Ma [21, 29, 30,8, 31]. YTS eruptive activity

spanned for more than 16 million years, most of the YTS

flows were emplaced over a period of 5 million years from ~

87 Abdul-Hamid Malek et al.: Cenozoic Eruptive Stratigraphy and Structure in Taiz area of Yemen

31 to ~26 Ma [27, 31, 13]. During this intense period of YTS

volcanism, most of the flows emplaced were of

extraordinary size, commonly exceeding 200 to 2,000 km in

volume. It covers many thousands of square kilometers [27]

and traveled many hundreds of kilometers from their linear

vent systems. The YVS include late Miocene-Recent

volcanic rocks, which concentrated between 10 Ma and 5

Ma [25, 28]. They are seen as more restricted and

disconnected occurrences above YTS in Marib graben and

sporadically along the Gulf of Aden from Bab Al Mandab to

the Qusay,ar – Sayhut area and covering a total area of about

9 km2 [18] (Fig. 1). Several Suites of Tertiary plutons

(named after locations at which they are exposed) intrude

the rocks of volcanic sequences [32, 33] and these events

have taken place during 22 to 21 Ma [34-36]. These silicic

intrusives are represented by granite, granodiorite and

syenite bodies emplaced in the form of plugs, domes, dyke

swarms, and composite stocks, associated with explosive

ash-flow [15, 20]. The silicic plutons are widespread but are

more abundant in the south and along the western

escarpment slopes of the plateau towards the Red Sea border.

The plutons have alkaline or peralkaline affinity and are

produced by fractional crystallization from basic magmas

[34, 37, 38].

They are coarse to medium grained, massive, and

remarkably discordant showing sharp contacts with the

country rocks. Angular inclusions of the basalt country rocks

are also noticed at places. The largest concentration of

plutons occurs along the western escarpment of the uplifted

region.

During and after volcanism of the YVG, the area was

subjected to block faulting and tilting. The processes of

uplifting and erosion that followed led to the exposure of

older rocks in the surrounding volcanic rocks.

3. Study Area

The Taiz area located between latitudes 13˚30' N and

13˚45' N and longitudes 43˚45' E and. 44˚15' E, occupies the

southern portion of a large physiographic province known as

the Yemen Highlands. It comprises dominantly of volcanic

rocks resting nonconformably upon sedimentary rocks

dominated by Cretaceous sandstone of Tawilah Group (Kt)

(Fig. 3a and b). Numerous intrusions in the form of granitic

rock bodies (Tg), mafic and silicic dykes are intruded into

the sandstone and volcanic rocks. The abundant volcanic

and intrusive rocks within the Taiz area suggest that the

region was a magmatic center during Cenozoic extension.

Many of volcanic rocks of Taiz area are largely obscured

by urbanization and surface sediment deposits. Each

eruption led to extrusion of a voluminous pyroclastic-flow

sheets interbedded with lava flows. The time gap between

successive lava flows was conducive for the development of

soil as witnessed in the occurrence of palaeosols at some

places. Each group of flows exhibits distinctive lithological

and stratigraphic characteristics and separated from one

another by well-developed unconformities. In some places

this succession becomes more elusive owing to lateral

changes within the volcanic stratigraphy as a result of flow

on the irregular surface, interfingering of

coeval/contemporary volcanic units erupted from different

centers, and pre-, syn- and post-volcanic block faulting. The

majority of volcanic succession was later affected by

extensive magmatism characterized by granitic intrusions

and the intrusion of mafic and felsic dykes. All the volcanic

rocks show signs of post-eruptive alteration. At places,

alteration has resulted in the complete replacement of all

primary mineral phases by secondary mineral assemblages.

The effects of alteration are remarkably seen in vesicular and

brecciated lava flows. Subsequent fluvial erosional

processes have deeply dissected the volcanic field providing

excellent exposures of several volcanic rocks. The best

exposures are seen along road cuts, quarries and stream

valleys, where outcrops are large and fresh.

3.1. Cenozoic Geology of Taiz Area

The Cenozoic volcanic stratigraphy of Taiz area is

composed of relatively younger rocks resting on Cretaceous

sandstone (Tawilah Group). The sequence is rocks

represented by a series of mafic and silicic rocks ranging in

age from the Oligocene to the lower Miocene [18] (Fig. 3a).

The complete stratigrahic section is divided into five units

(Fig. 3b), with well defined boundaries, representing

different volcanic eruptions. From base to top the Cenozoic

volcanic section in the study area comprises: (1) lower mafic

sequence (Tb1), (2) lower silicic rocks sequence (Tr1), (3)

middle mafic sequence (Tb2), (4) upper silicic volcanic

rocks (Tr2), and (5) upper basalt flows (Tb3). The

stratigraphic section includes several important sediments

containing abundant volcaniclastic components derived

from surrounding volcanic rocks.

3.1.1. Lower Basalt Sequence (Tb1)

Stratified and massive on a minor scale, basaltic lavas of

several hundred meters thick, frequently extruded primarily

through the feeder dykes paralleling the axis of the Red Sea.

constitute the lower basalt sequence (Tb1) of the Taiz area.

It is most voluminous rocks of the YVG in Taiz area that

rest unconformably on an irregular topographic surface of

Tawilah Group. The unconformity can be seen on the

western rim of the mapped area (Fig. 4a). Contact with the

underlying rocks is generally obscured in most part of the

studied area, but the upper contact with lower silicic rocks

is well exposed and sharp, commonly irregular in most of

the localities. At places, the Tb1 flows occur as cap above

granitic rocks of the Saber Pluton. (Fig. 3a) shows the flow

that is forcefully intruded by Saber granite pluton with no

effects of thermal metamorphism. The thick stack of Tb1

pile (cumulative thickness of about 600 meters) comprises

either multiple or single cooling flow interbeded with

centimeter to several meter thick volcaniclastic basic rocks

of various kinds. Due to differential intensity of weathering,

the colour of basaltic lavas and their volcaniclastic inter-

calations varies from dark gray in fresh surface, to

Earth Sciences 2014; 3(3): 85-96 88

chocolate brown or dark reddish brown, often hampering

identification of the actual rock composition in weathering

surface. These rocks consist of semiconsolidated deposits

of mafic ash, air-fall scoriaceous deposits and tuffaceous

sediments. Many glass fragments, not greater than I.5 cm

are observed in the matrix of the pyroclastic units. The Tb1

flows exhibit well developed colonnade and entablature

structures with low proportion of vesicles. The lower (basal)

entablature zone commonly grades upward into a

colonnade in some flows.

Figure 3. (a). Geologic map of the study area, indicating rock types and major faults (modified after Kruck and Schaffer 1991). (b): northwest-southeast

cross section through the study area, illustrating relationship between rock units and normal faults.

The rocks are frequently strongly fractured with fracture

spacing from a few to ~15 cm. The calcite veins are

occasionally present. Texturally, the lower basalt flows

consist of two different varieties; (1) the more widespread

fine grained nonporphyritic, locally containing vesicles and

amygdules of secondary minerals (Fig. 4b), (2) porphyritic,

where the plagioclase and/or olivine phenocrysts in some

specimens are distinguishable megascopically. Plagioclase

phenocrysts are euhedral to subhedral, zoned and

occasionally clustered into aggregates thus exhibiting

glomeroporphyritic texture and some phenocrysts define a

flow fabric foliation. Larger six-sided olivine phenocrysts

in the fine groundmass are typically highly fractured with

some zoning (Fig. 4c). These are often altered at their rims

and fractures into iddingesite. Olivine is present in some

thin sections and absent in others. The fracturing and

breakage of some phenocrysts suggest turbulent transport.

Euhedral pyroxenes are also present as phenocrysts and in

the groundmass. In places and particularly in the

northeastern part of the map area, the Tb1 are scoriaceous

and vesicular and include pyroclastics beds and lenses. The

brecciated and vesiculated zones are visible along fault

89 Abdul-Hamid Malek et al.: Cenozoic Eruptive Stratigraphy and Structure in Taiz area of Yemen

plane. Mineralogy of some dyke rocks are similar to basalt

flows of Tb1 and the dykes cut the sequence of older rocks

in the area and may have served as feeders for the mafic

flows within the sequence (Fig. 4d). The weathering and

alteration of rocks toward upper contact zone hamper the

study of volcanic structures at the top of the basalt flows.

Figure 4. (a) A view showing Tawilah sandstone (lower part of the photo),

unconformably overlaid by basalt flows (upper part of the

photograph) .Note swarms of basic dykes forming feeder to basaltic lavas

of the Lower basalt sequence,(c) Photomicrograph of olivine phenocrysts

showing zoning, coating in a finer groundmass under normal light,(d)

View of ~1 m wide feeder dyke of basalt connecting with 4 m thick sill

sandwiched between two basalt flows.

3.1.2. Lower Silicic Rocks Sequence (Tr1)

Figure 5. (a) View of rhyolite lava flows with well-developed colonnade

columnar jointing in the upper flow,(b) View of an infilled channel that has

cut down through volcanic tuff.(c) Highly vesicular upper part of the rhyolite flow,(d) Photomicrograph of sanidine phenocrysts in a finer

ground mass (crossed nicols).

The Tb1 sequence is overlain by an extensive section of

silicic volcanic rocks (Tr1) forming plateaus and rarely

small hills consisting of varied assortment of lithologies

including rhyolite flows, ignimbrites, welded ash,

volcaniclastic breccia, and random pumice and obsidian.

The higher amounts of volcaniclastic rocks indicate that

initially volcanism of silicic volcanic rocks was more

explosive. The Tr1 either rest directly on the basaltic rocks

or locally upon a 0.5 – 5 m thick clastic sediments and

paleosol. Contacts with the underlying and overlying basalt

and /or clastic sediments are sharp but slightly irregular.

The Tr1 varies in their appearance from multi-sheet

stratifications to domal mountains and hills with elevation

up to 200 m from the local base level and extends to several

kilometers (Fig. 5a). These rocks at places extend through

canyons (Fig. 5b). The tabular bedding are interpreted to

represent ash-fall deposits. Some exposures of rhyolite

flows display colonnade shrinking joints and often form a

regular pattern of parallel discontinuities (Fig. 5a). The

jointed rhyolite has been quarried for local building

purposes. It exhibits spheroidal, exfoliation, blocky and

cavernous weathering features. The flow beds occur in

between two pyroclastic layers and can be subdivided into

three parts based on vesicle distribution patterns. The basal

part is massive that noticeably lacks vesicles and is

irregularly jointed. It is overlain by about 2 m thick bed

containing few numbers of large ellipsoidal vesicles. This

bed is overlain by about 1 m thick bed containing vesicles

ranging in size from a few millimeters to several tens of

centimeters (Fig. 5c). These vesicles and cavities are

sometimes filled by secondary alteration minerals such as

calcite and chalcedony. At places, the vesicles are filled by

chalcedony forming thunder eggs structures. The size of

concentric or star-shaped thunder eggs varies from 1 cm to

8 cm. The ryholite lava flows are generally phyric

containing phenocrysts of quartz and sanidine, commonly

fractured or fragmented, suggesting that they were broken

during emplacement (Fig. 5d). Quartz and feldspar

phenocryst, commonly are clustered in aglomeroporphyritic

texture. Flow-bands clearly indicate an extrusive mode of

emplacement. The rhyolite quarry, located at Al-Masnah

village along Al-Steen Road, about 15 km north of the Taiz

city, from bottom to top consists of sequence of alternating

layers as follows: (1) whitish yellow of welded pyroclastic

rhyolite, ranging from fine-grained ash to lapilli, (2) highly

vesicular, whitish yellow pumice. (3) blood-red, massive,

fine grained, highly welded ignimbrite with flow banding

structure and (4) reddish yellow, colonnade columnar

jointing rhyolite. Contact relations between the pyroclastic

rocks and rhyolite indicate that they were emplaced at least

in part simultaneously. The volcaniclastic rhyolites show

both vertical and lateral diversity in terms of color,

pyroclast lithology and degree of alteration within

individual units. The basalt fragments generally occur as

dark brown, round spots or elongated fragments within the

flow, but also larger, sheet-like fragments are occasionally

observed. Tuff sheets are exposed at different levels within

silicic volcanic rocks, more commonly near the bottom

than at the top of the sequence. A relatively persistent zone

of volcaniclastic breccia is sandwiched and found at about

100 m below the top of the horizon. Tuff sheets in most

cases rests with apparent conformity and is essentially

coextensive with the underlying and overlaying rocks. The

tuffs are mostly welded, felsic, creamy white to light gray

Earth Sciences 2014; 3(3): 85-96 90

in color and locally occur as reddish rock. In some

localities tuff sheets tend to weather into unconsolidated

soil. They are composed of angular mineral grains in a fine

fragmental matrix. Much of the tuffaceous material in this

unit contains lithic fragments (up to 1 m in diameter)

derived from older local units.

3.1.3. Middle Basalt Sequence (Tb2)

Figure 6. (a)View of sharp contact between acidic volcanic ash (lower)

and middle basalt sequence, (b) Four simple sheet lava flows of Tb2, note

the thin saprolitic bole at the top of each basalt flows, (c) Flow structure

in basalt (d) View of volcaniclastic conglomerate with porphyritic basalt

fragments in fine grained tuff matrix, (e) View of volcanic bombs

embedded in fine-grained basalt, (f) Colonnade columnar jointing, (g)The

entablature columnar jointing in lower flow of Tb2.

Middle basalt sequence (Tb2) stands out in marked

contrast to the much thicker flows of the Tr2 sequence (Fig.

6a). It consists of at least four flows, which vary in

thickness from 1 to 20 m (Fig. 6b). At the Al-Steen road,

the total thickness of Tb2 is at least 100 m. Entire sequence

of Tb2 thins westward. The basal contact is sharp and

planer and overlies an extremely weathered flow top of the

underlying flow Tr1 (Fig. 6b). This is marked by a

reddened weathering band ~ 50 cm known as saprolitic

bole (paleosol). The red color indicates that the iron in the

rock is oxidized. In some places, the weathering bands

grades downward from red mudstone into debris zone

dominated by basalt core stones produced from underlying

flow. In most hills, Tb2 flows cap stratigraphic unit of YVG.

These basaltic rocks are mainly aphyric, although

plagioclase and olivine porphyritic types are also

commonly observed (Fig. 6c). N-S to NW-SE trending

feeder dykes located along Al-Steen road, appear to be a

large vent of basalt flows. The volcaniclastic conglomerate

measuring up to 6m thick intercalated between two basalt

flows (Fig. 6a) is observed at Al-Gibarah village. The

volcaniclastic conglomerate is poorly sorted clasts

consisting of angular, subangular, to subrounded vesicular/

amygdaloidal to non-vesicular basalt lava clasts, and the

latter range in size from those of pebbles and to cobbels.

(Fig. 6d). Volcanic bombs are observed within some

exposures exhibiting vesicle-poor cores whilst having

highly vesicular margins (Fig. 6e). The columnar jointing

structures known as colonnade and entablature are common

structures observed in Tb2. Colonnade structures

commonly occur in the upper part of basalt flow, and

consist of relatively well-formed 10 - 50 cm basalt columns

which are typically oriented perpendicular to the base of

the flow (Fig. 6f). The entablature zone exhibits small (30

to 10 cm in diameter) irregular columns, which may form

radiating or fanning patterns, or otherwise deviate from an

orientation perpendicular to the base of the flow (Fig. 6g).

In many basalt flows, entablature grades upward into an

upper colonnade zone. Hollow spherical to oblate geodes

occur in large quantities within Tb2, particularly in the

northeastern part of the study area. The geodes range in size

from a few to tens of centimeters, have an outer shell of

chalcedony, followed inwards by white color lined agate

and finally euhedral, colorless quartz and /or calcite.

3.1.4. Upper Silicic Volcanic Rocks (Tr2)

A well-preserved upper silicic volcanic rocks (Tr2) were

found in fault contact of Saber pluton in the form of

isolated domal mountains and plugs, rising to an altitude of

about 250 m above local base level. At places such as

Gabal Ruthagah and Adenah, Tr2 overlies Tb2 which can

be laterally traced for several kilometers. The domes occur

in the forms of conical, hemispherical and nearly flat tops,

with varying slopes. The slope regions consist of different

size blocks, up to several meters in their diameters (Fig. 7a).

The outer surfaces are relatively rough owing to fracturing

and exfoliation phenomena. The rhyolitic rocks display

lateral changes in their colour and hardness due to the

changes in their proportion of pyroclastic materials and

rock texture. The pyroclasts-poor rhyolite is characterized

by abundant feldspar and quartz phenocrysts in fine-

grained matrix in comparison to pyroclasts-rich rhyolite. A

number of well-defined ridges and knobs of

pyroclasts-poor rhyolite observed at Gabal Al-Birarah are

accentuated by weathering. Some ridges with medium- to

coarse- grained massive rhyolites exhibiting spheroidal

weathering and feldspar phenocrysts may represent

subvolcanic dykes. In some places, as seen in the road cut

below Gabal Al-Garah ridge, a clastic immature sediment

91 Abdul-Hamid Malek et al.: Cenozoic Eruptive Stratigraphy and Structure in Taiz area of Yemen

zone (Fig. 7b) separates Tr2 from Tb2. This zone may

probably represent quiescent period of volcanic activity

during which rock weathering and erosional processes lead

to the formation of poorly bedded sedimentary horizons.

The presence of this clastic unit in some localities and

disappearance in others may be attributed to the significant

variation in the topography of older basalt sequence.

Figure 7. (a) View of southeast side of Cairo Castle cone , composed of

mixtures of rhyolite lava flows(upper) with ignimbrite and tuffs(lower), (b)

View showing Tr2 rhyolite (upper part of the photo), underlain by mixture

of clastic, immature sediments (lower part of the photo), (c) View of

accretionary lapilli in fine grained tuff, Thoapat area, (d)

Photomicrograph showing granophyric texture, consisting of two zones of

quartz which fill most of an original simple twinned K-feldspar phenocryst

(Crossed nicols), (e) Photomicrograph showing spherulite texture

nucleated around fine grain (Crossed nicols), (f) Photomicrograph of

welded pyroclasts consisting mainly of altered quartz and feldspars..

Topographically elevated parts are the probable sources

for these coarse clastic sediments. The lower contact of Tr2

over the underlying Tb2 at Gabal Kilabah shows a red

coloured zone which possibly represent a hydrothermally

altered lateritic – horizon. Occurrence of erosional surface

between the rhyolitic rocks and underlying basalt flows

implies discontinuity of eruption of rhyolite and basalt. The

Tr2 show varying colors including white, yellow, pink and

greenish grey. In the quarries and road cuts, located in

different localities within Taiz city, the Tr2 comprises

alternating layers or beds of welded and unwelded

volcaniclastic units, varying in bedding character, grain size

(ranging from fine-grained ash to lapilli), and depositional

structure. Volcaniclastic units contain different sizes of

lithic rhyolite, obsidian and basaltic clasts pointing to the

violent explosive nature of the silicic volcanism. In many

cases, pyroclastic flow deposits are internally massive or

stratified. The high concentration of accretionary lapilli in

fine grained tuff, bedded with white to light gray, friable,

fine grained tuff as noticed in Thoapat area indicates that

water was occasionally present in the eruptive cloud in

subaerial deposition environment (Fig. 7c). Microscopic

study of the flow revealed the occurrence of the Tr2 as

altered rock. A variety of textures have been observed in

the the rhyolitic flows including porphyritic, granophyric

and spherulitic. The porphyritic textured rhyolite contains

phenocrysts of sanidine, plagioclase, and quartz embeded

in highly altered fine grained groundmass. The groundmass

is locally vitrophyric but generally devitrified. The

granophyric texture radiates out from feldspar grains (Fig.

7d). Spherulites occur mostly as isolated and frequently

nucleated on existing fine grains (Fig. 7e). The

volcaniclastic units are made of detritus consisting of

mineral grains, rock fragments and glass shards. Quartz and

feldspars are common grains occur as irregular angular and

rectangular shaped grains (Fig. 7f). In places, the silicic

volcanic rocks have been altered to bentonitic clay minerals

and zeolites particularly in volcaniclastic units where the

rhyolitic rocks are highly fractured. The presence of

younger rhyolite dykes dissecting Tr2 suggests the

extrusion of lava in the form of series of domes and flows

rising more mainly along fractures.

3.1.5. Felsic Plutonic Rocks (Tg)

Felsic plutons of varying dimensions are found intruding

into the older volcanic rocks (Tb1, Tr1, Tb2 and Tr2) in

Taiz area. Saber Mountain with an altitude of 3000 m. a.s.l.

exposed in the southern border of Taiz city represents a

largest pluton. Some of the smaller plutons viz., Daneq and

Gabal Habashi rise to altitudes of 1650 and 2300 m. a.s.l.

Other unnamed small masses or dyke-like intrusions such

as those at Wadi Al-Dahi (40 m thick) are considered as

small offshoots sent from the main pluton into the adjoining

volcanic rocks.

Figure 8. (a) View showing sharp contact between granite and older

basalt (Tb1). (b) View of Saber mountain escarpment showing granite

crowned by older basalt and in fault contact with rhyolites.

Obviously these Tertiary granite plutons represent the

coeval magmatic phase with the different intrusions,

Earth Sciences 2014; 3(3): 85-96 92

located in the western part of Yemen. Plutons vary in

composition from alkali granite to syenogranite [15, 38],

however to simplify the terms for this study, all these

variable compositions are referred to as "granites". The

granite plutons occur as stocks, typically show sharp and

discordant contacts with adjoining volcanic rocks,

indicating intrusion into brittle and cooler crust (Fig. 8a).

Both Saber and Daneq plutons have oval shape, elongated

in E-W direction for a distance of 9.5, and 2.2 km

respectively, with elevations ranging up to 1.4 km from the

local baseline and corresponding to a total surface area of

approximately 70 km2 (Fig. 8b). The granitic rocks are

grayish white in colour, massive, locally strongly fractured

and spheroidally weathered and occur in the form of large

blocks. The blocks have undergone chemical weathering as

evidenced by the formation of clay minerals at the margins.

The grain size of granites ranges from medium to coarse.

The radiometric dating carried out on the Tertiary granites

have yielded an intrusion age of about 21.9 ± 0.7 Ma [33].

3.1.6. Upper Basalt Flows (Tb3)

The Upper basalt flows (Tb3) are the youngest Tertiary

volcanic rocks which are confined to the southeastern part of

the mapped area in the form of a few isolated small outcrops,

conformably capping the Tr2 (Fig. 9a). It includes the

basaltic rocks that makes up the predominant flows

intercalated by andesite, mafic conglomerate and tuff layers

or as dykes cutting across the volcanic rocks and granite

intrusions (Fig. 9b). In the Naqil Al-Epil area, the Tb3 basalt

interfingers with the underlying Tr2 rhyolite. Absence of

erosional surface between basalt lava flows and pyroclastic

units suggest that there was no major hiatus during the

formation of the volcanic center.

Microscopic observations show that most of the Tb3 are

fine grained, nonporphyritic and rarely contains plagioclase

phenocrysts (Fig. 9b).

Figure 9. (a) Top of Naqil Al-Epil hill exhibiting the interlayering of

volcanic tuff and basalt lava flows (Tb3). (b) Photomicrograph of fine

grained basalt containing plagioclase phenocrysts.

3.1.7. Dykes and Sills

Multiple generations of basic and silicic dykes show

NW-SE, NNW-SSE, NE-SW and E-W orientations and have

intruded into Tawilah sandstone and volcanic succession in

Taiz area. It appears that the intrusive material fills a set of

subvertical tensional fissures produced during the rifting

process. The close spatial relation of various dykes with five

volcanic successions described above indicate that the dykes

are injected during different phases coeval with mafic and

felsic igneous products. Their relative ages can be

established from cross-cutting relationships between them.

Sometimes the dykes are extremely difficult to distinguish

from similar rocks, but in fresh exposures, as in road cuts

and quarries, the dykes are readily visible because of their

strong contrast in color and texture with the similar volcanic

rocks which they intrude (Fig. 4d). The dykes can have a

planar form or very irregular margins. The mafic dykes are

contemporaneous with Tb1, Tb2 and Tb3. These dykes are

dark gray, porphyritic to fine-grained, sometimes contain

plagioclase and olivine phenocrysts. The dykes are generally

thin (< 2m), although the dykes exceeding ten meters are

also observed in Hagdah and Al-Manakh Mountains. Dykes

appear as cliffs or dip steeply in NNW to NE directions and

appear to be controlled by rifting joint or fault trends. The

contact is sharp and no visible chilling effects are observed

especially in thin dykes. Some dykes preserve flow fabrics.

Mafic dykes arranged in parallel to subparallel groups

representing "swarms", intrude cretaceous sandstone and

volcanic succession in the study area. In some localities, the

columnar sheet basalt in the succession, has not been

extruded as lava, but injected between the basalt flows as

subhorizontal basaltic sills (Fig. 4d). The sills are fed and

linked by dykes (Fig. 4e). Few meters long horizontal to

subvertical offshoots originated from several mafic dykes

are seen. These offshoots are commonly found in the dykes

emplaced adjacent to the sandstone.

The silicic dykes intruded into the volcanic succession

show dominant orientations along N–S, NNE-SSW and

NW-SE directions. The dykes are 1 to 12 m thick, although

the thicknesses of individual dykes are seldom constant even

for a short distance. Many of them extend to several

kilometers. Some rhyolite dykes form prominent, blocky

ridges, and others are less resistant to weathering than the

surrounding rocks and form deep trenches, where they show

a prominent horizontal columnar jointing, polygonal

columns mostly being pentagonal or hexagonal in cross

sections. These dykes are white, yellow, and greenish white

to reddish in color, generally distinctly porphyritic with

feldspar phenocrysts. Many aplitic dykes were observed

cutting the volcanic succession around Saber granite pluton,

and these do not cross cut granite thus indicating them to

be off shoots from a feeder pluton.

3.2. Structures

Geological map of Taiz area (Fig. 3a) reveals that the

distribution of the various rock types and geomorphic

features appears to be significantly influenced by NNW-SSE,

NE-SW and E-W trending normal faults created by

extension associated with the opening of the Red Sea and

Gulf of Aden [e.g.(39,40)]. The marking of the spatial

distribution of the faults was initially achieved by tracing

linear elements on ETM+ satellite image and earlier

geological maps. Ground checks were made wherever

necessary. The faults have linear or curved shape, with

terminal segments showing a systematic tendency to change

their strike directions. Cross-cutting relationships between

93 Abdul-Hamid Malek et al.: Cenozoic Eruptive Stratigraphy and Structure in Taiz area of Yemen

the two or more of normal faults display that the NW-SE to

NNW-SSE trending fault system is relatively older than the

E-W faults system. The faults have a low to high vertical

displacement, characterized by the presence of fault scarps,

dividing the rocks of the study area into tilted blocks that

have relatively steep slopes toward east and north.

Movement took place on a vertical or inclined surface but

assumed a listric form in the underlying rocks. The listric

normal movement caused the rotation of the hanging wall

block (Fig. 10 a and b). The magnitude of the throw varies

and the maximum vertical displacement is seen to the north

of Saber pluton wherein the throw, on the eastern face,

measures about 1100 m (Fig. 8b). The grinding between the

two fault blocks caused the rock to break up and become less

resistant which upon erosion, led to the formation of the

trenches. The fault blocks are commonly obscured by

weathering and wadi sediment accumulations but where

visible, they show cataclastic texture. The fault gouge,

breccias (consolidated and unconsolidated forms) and

slickensides, which carry striations on their surface, are the

major indicators observed along fault-line scarps, which can

be distinguished from the host rock on the basis of texture

and color (Fig. 10c). The fault scarps on volcanic rocks

generally provide the most reliable indications of relative

age of fault formation, whereas the fault scarps on granite

rock are degraded owing to erosion. The most obvious

structural feature of the area is the moderately tilted volcanic

sequence that dips in several directions, dominantly towards

east, defining different trends of normal faults (Fig. 3b). The

multiple downthrows of faults toward each other at several

places form small grabens (Fig. 10d).

The first major fault system trending in NW-SE to

NNW-SSE direction, extends parallel to the SW Saber

master fault. The fault system consists of a series of parallel

faults defining mountains and depressions topography. This

direction corresponds to the volcanic emplacement, and Red

Sea rifting. From the geological map (Fig. 3a), it is evident

that the western margin contains stratigraphy that cannot be

recognized in the east which is controlled by NW-striking

faults. The second major fault trend is parallel to the

southeast Saber master fault. This fault system bounds the

Saber mountain to the south east and run in the NE-SW

direction parallel to the Gulf of Aden axis. The third trend of

normal faults is parallel to the major fault located North

Saber mountain, which extends several kilometers with

general E-W to ESE–WNW strike. They dip either to the

north or south. Many of these faults are well exposed and

contain well-developed striate and kinematic indicators.

Several of smaller offset synthetic and antithetic normal

faults, most commonly exposed in the central part of the

mapped area, often produce several small half-grabens.

Figure 10a shows good examples of such antithetic normal

faults in the central part of the area. The extensive dip slopes

and tilt direction of the rhyolites toward north at northern

border of Saber mountain may be related to the master fault

downthrown mostly on its north side, here named the North

Saber master fault. Displacement along this fault decreases

westward where the rhyolite volcanic sequence is in direct

contact with lower basalt sequence.

Figure 10. (a and b) Rhyolite dome dissected by two normal south-dipping

antithetic listric faults parallel to the Saber master fault causing rotation

of the hanging wall block, (c) View shows normal fault separating the

Tawilah sandstone (in the left) and the Tb1 basalt (in the right). The inset

show fault breccia and gouge along a normal fault, (d) View looking east,

from the top of Naqil Al-Epil shows two opposite displacement normal

faults forming small graben. Dotted lines show conformable contacts

between volcanic units.

4. Conclusions

The present study covers Cenozoic stratigraphy and

structure of Taiz area of Yemen based on field geology.

Magmatism in the study area is related to the continental

rifting and break up processes associated with Afar mantle

plume. The study provides for the first time the volcanic

stratigraphy and structure of the Taiz area which have

important implications concerning the geodynamics and

magmatic evolution of the rift.

The excellent exposure of the Tertiary magmatic rocks

provides an unique opportunity to study different volcanic

succession units and related intrusions in the Taiz area.

Detailed studies carried out on the stratigraphy and

structures in Taiz area led to the following conclusions:

1. The study area in and around Taiz city is considered to

be a part of an YVG, where the basalt-rhyolite association

and accompanied volcaniclastic units have formed the

major mountainous backbone of the area. The bimodal

distribution of silica is attributed to systematic variation in

melting depth over time.

2. The past and present studies reveal that the volcanic

rocks in Taiz area closely resemble in their extension and

thick accumulation to the volcanic products of an

intra-continental rift setting. In this setting large effusive

volcanoes extruded lavas that have been inter-fingered and

injected in the forms of plutons, dykes and sills.

Earth Sciences 2014; 3(3): 85-96 94

3. Based on the field, petrographic and radiometric

evidences, it can be deduced that the volcanism in Taiz area

was active for more than 10 Ma. The sheet-like distribution

of volcanic rocks indicates that the entire succession is an

accumulation zone erupted through three mafic and two

silicic phases of volcanism instead of individual volcanic

edifices. Early volcanism was dominated by the effusion of

mafic lava flows (Tb1), beginning at about 31 Ma followed

by silicic volcanism (Tr1) at about 29.7 Ma. Later eruptions

of voluminous mafic and silicic lava flows (Tb2 and Tr2

respectively) continued till the emplacement of Saber

granite which is estimated at about 21 Ma, followed by Tb3

mafic eruptions. The stages in the evolution of volcanism

and associated landforms are schematically depicted in Fig.

11a – g.

4. Each eruption started with the formation of pyroclastic

deposits and ending with the relatively quiet effusion of

magma to form lava flows or domes.

5. Source vents for the lavas especially basaltic lavas are

difficult to recognize, but abundant dikes suggest that many

lava flows may have been erupted from fissures rather than

spot sources.

6. The common interbedding of basalt and rhyolite

indicates that such magmas were available at the same

place contemporaneously. Field observations support the

view that two magmas of highly contrasting compositions

coexisted at least for limited periods and were erupted

sequentially throughout the geologic time.

7. In the study area, occurrence of pillow lava is no

where reported. Further, the presence of columnar jointing

and weathered flow tops are good signs to infer the

existence of subaerial conditions during the eruption

history of the Taiz lavas and the accumulation of volcanic

products.

8. The volcaniclastic successions identified in the Taiz

area documents a complex eruption, transportation and

depositional history, where primary pyroclastic units

interbedded with eruption flows show saprolitic bole and

weathered flow tops with uncommon inter-eruption

epiclastic succession. The large volume of pyroclastics in

the entire Taiz succession is interpreted to be deposited by

debris flows produced by repeated pyroclasts flow eruption.

The rare occurrence of non-volcanic debris in most of

volcaniclastic rocks indicate that the source area of these

debris flows is volcanic in origin.

9. The volcanic phases are intermittent with quiescent

periods during which rock weathering processes and

redeposition, led to the formation of poorly bedded

sedimentary horizons.

10. Final stages of igneous activity involved the

emplacement of Saber granite plutons and small intrusive

bodies at about 21 Ma.

11. Multiple generations of dykes, normal faults and

deformed volcanic succession show very wide variation in

orientation, but with a bias in favor of NW-SE and NE- SW

trending faults and dykes.

12. The mapped area is characterized by the focus of

volcanic centers related to the rhyolite volcanic sequence.

Evidence for this is found in the form of columnar jointing,

rhyolite domes, abundant pyroclastic material and rhyolite

dykes cutting lower basalt sequence, and the remarkable

change in the thickness of volcanic sequence.

95 Abdul-Hamid Malek et al.: Cenozoic Eruptive Stratigraphy and Structure in Taiz area of Yemen

Figure 11. Schematic model diagrams illustrating the evolution of the Taiz

Area, (a) eruption of the thick bile of Tb1 basalt above the Tawillah

Sandstone group, (b) Extrusion of rhyolite with explosion led to extrusion of

volcaniclastic materials with thyolite flows (Tr2), (c) Eruption of the Tb2

basalt over saprolitic bole (paleosol) weathered surface, (d) Extrusion of

rhyolite flows and volcaniclastic materials in the dome complex, (e)

Emplacement of the Saber granite and it's associated offshoots, (f )

Eruption of the youngest basalt as cap above Tr2 silicic rocks and locally

above Tb2 basalt and, (g) Normal faulting, where the granite is exposed as

a result of the faulting and erosion of the footwall block.

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